Up to now, in an electric power steering system of a vehicle, a control device has been known which controls an assist torque output by a motor on the basis of information such as a steering torque.
For example, a device disclosed in JP-A-2013-126822 estimates an axial force to be applied to a steering shaft on the basis of detection values of a torque sensor that detects a steering torque, a current sensor that detects a real current flowing in the motor, and a motor rotation angle sensor that detects a rotation angular velocity of the motor. The estimated axial force is input to a motor control signal output unit, and a target assist value is changed on the basis of a determination result using the estimated axial force to improve a steering feeling.
In the device of JP-A-2013-126822, a detected value of a real motor current by the current sensor well reflects a real motor output, but has the potential to include a high frequency component such as sensor noise. Therefore, when the axial force to be applied to the steering axis is estimated on the basis of the detected value of the current sensor, the use of the estimated value including a high frequency component for calculation which is a basis of an assist control of the electric power steering system has a problem in reliability.